The invention relates to improved drilling fluids used in the drilling of subterranean oil and gas wells as well as other drilling fluid applications and drilling procedures. The invention is particularly concerned with non-polluting, minimally toxic drilling fluids.
In rotary drilling there are a variety of functions and characteristics that are expected of a drilling fluid ("drilling mud" or simply "mud"). The drilling fluid is expected to carry cuttings from beneath the bit, transport them up the annulus, and permit their separation at the surface while at the same time the rotary bit is cooled and cleaned. A drilling mud is also intended to reduce friction between the drill string and the sides of the hole while maintaining the stability of uncased sections of the borehole. Likewise the drilling fluid is formulated to prevent unwanted influxes of formation fluids from permeable rocks penetrated and likewise to form a thin, low permeability filter cake which seals pores and other openings and formations penetrated by the bit. Finally, the drilling fluid is used to collect and interpret information available from drill cuttings, cores and electrical logs.
Drilling fluids are typically classified according to their base material. In water based muds, solid particles are suspended in water or brine. Oil can be emulsified in the water. Nonetheless, the water is the continuous phase. Oil based muds are exactly the opposite. Solid particles are suspended in oil and water or brine is emulsified in the oil and therefore the oil is the continuous phase. The final class of drilling fluids are pneumatic fluids in which drill cuttings are removed by a high velocity stream of air or natural gas.
On both offshore and inland drilling barges and rigs, drill cuttings are conveyed up the hole by a drilling fluid. Water based drilling fluids may be suitable for drilling in certain types of formations; however, for proper drilling in other formations, it is desirable to use an oil base drilling fluid. With an oil base drilling fluid, the cuttings, besides ordinarily containing moisture, are necessarily coated with an adherent film or layer of oily drilling fluid which may penetrate into the interior of the cuttings. This is true despite the use of various vibrating screens, mechanical separation devices and various chemical and washing techniques. Because of pollution to the environment, whether on water or on land, the cuttings cannot be permanently discarded until the pollutants have been removed.
Another method to accomplish the pollutant removal has been placing the screened cuttings in a standpipe or other vessel filled with sea water and periodically skimming off the layer of displaced oil as it rises to the surface in the vessel. Another method attempted is burning, i.e., oxidatively incinerating, the oil from the cuttings. Still another method is physically transporting the oily cuttings to a remote site for subsequent disposal. In each instance the method of disposal of the cuttings has proved ineffective and inefficient.
The problems associated with the environmental compatibility of drill cuttings, and the chemicals contained therein, has long been recognized as a problem in the oil and gas exploration industry. Typically the approaches for solving the environmental compatibility problems have involved the physical treatment of the drill cuttings, see for example U.S. Pat. No. 4,208,285 wherein an apparatus is provided for removing volatile materials from drill cuttings by vaporizing the materials on the cuttings in a non-oxidative atmosphere and U.S. Pat. No. 4,387,514 which provides a method and apparatus for drying oil well drill cuttings to eliminate pollution causing organic materials from the cuttings.
It is apparent to anyone selecting or using a drilling fluid for oil and gas exploration that an essential component of a selected fluid is that it be properly formulated to achieve the necessary characteristics for the specific end application. As stated hereinabove, the typical compositions include oil based muds, water based muds and pneumatic fluids. For purposes of this application, pneumatic fluids are not relevant. The vast majority of oil and gas exploration is done with water based muds. The primary reason for this preference is price and environmental compatibility. Oil based muds are substantially more expensive than water based drilling fluids, and are also environmentally incompatible. As a result, the use of oil based muds has been historically limited to those situations where they are necessary.
This long felt need in the oil and gas exploration industry for an environmentally acceptable drilling fluid which either is an oil based drilling fluid or performs as an oil based drilling fluid has now been achieved by applicants' invention. By use of applicants' invention and the use of synthetic hydrocarbons the functional characteristics of an oil based drilling fluid system are achieved while the environmental compatibility of conventional water based systems is attained. Such a result has until recently been thought theoretically and practically impossible.
Various advantages of using oil based drilling mud in the rotary drilling of boreholes in the earth have been known for some time. In summary, it can be said that under certain conditions, one or more of the following advantages make oil based muds more desirable than water based muds. Lubricity characteristics are excellent. Drilling fluids weighing less than about 8 pounds per gallon can be prepared with an oil based mud, and have proved advantageous in certain special drilling and coring situations. The penetration of the formation by water is avoided. Cores can be recovered in an in situ condition, and in the presence of hydrous clays and bentonites, no swelling or sloughing is experienced to reduce porosities or cause pipe sticking difficulties. There are fewer problems with hydrophilic shale formations drilled. Evaporite sections such as salt zones can be drilled with minimal leaching of the salt. Furthermore, oil based muds can be formulated to withstand temperatures up to 500 degrees Fahrenheit.
The principal difficulties encountered in the use of oil based drilling fluids are probably those of handling, fire hazard, environmental unacceptability and the intrinsic cost per barrel, since oil is much more expensive than water. Some materials which heretofore have been added to oils in order to produce satisfactory drilling muds have been of an intense black color, e.g., carbon black and asphalt of various kinds, and oilfield workers have a strong resistance to handling such materials. Not just any material can be added to an oil for the purpose of making a drilling mud, because the material to be added must not only increase the viscosity, though not to too great an extent, and decrease the fluid loss, but must also be dispersable in the oil, and above all must not be of such a nature as to clog the formations drilled through.
In addition, oil based muds offer potential advantages; better lubricating qualities, higher boiling points, and lower freeze points. Because of the cost of preparing an oil mud is always more than that of the same density water muds, the economic justification for selecting an oil mud must come from its superior performance under the particular conditions of use.
There is, however, strong sentiment that although the initial differential cost of oil based muds versus aqueous based muds favors aqueous based, as the well is drilled deeper and as drilling operations last longer, the oil based muds actually become more economical due to the fact that the constituents do not have to be replenished as often and the fact that the oil based muds are reusable for substantially longer periods of time.
As can be seen from the above, the development of a drilling fluid that exhibits desirable characteristics of both a water based and oil based drilling fluid has long been an unachieved goal of the oil and gas exploration industry. With the practice of applicants'invention this goal has been realized.